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Power vs lighting system and cable sizing factors

Short answer

Power system uses 3-phase 400 V, high inductive loads, larger cables, D-curve breakers; lighting uses 1-phase 230 V, steady loads, smaller cables, B/C-curve MCBs. Cable sizing factors: load current, current-carrying capacity, derating, voltage drop, short-circuit rating, protective device coordination, conductor material, and future expansion.

What is the main difference between power and lighting system? Describe different factors to be considered while determining the sizes of cables.

Power system vs lighting system

PointPower systemLighting system
LoadsMotors, heaters, ACs, machines, power socketsLamps, fans, small 5 A sockets
SupplyMostly 3-phase 400 VMostly single-phase 230 V
Current per circuitHigh (16 A and above)Low (6–10 A per circuit)
Cable size2.5 mm² and above1.5 mm² Cu typical
ProtectionMCCB/MCB (C or D curve), overload relays, startersMCB (B/C curve), RCCB
Load behaviourInductive, starting surgesMostly steady, fairly constant
WiringSeparate power DB and circuitsSeparate lighting DB and circuits

The two are kept on separate circuits (often separate DBs) so that a fault on a machine does not black out the lighting, and so that energy can be metered separately.

Factors in determining cable size

  1. Load (design) current and type of load.
  2. Current-carrying capacity of the cable from tables.
  3. Derating for ambient temperature, grouping and laying method.
  4. Permissible voltage drop and length of run.
  5. Short-circuit current and fault duration.
  6. Rating of the protective device (Ib ≤ In ≤ Iz).
  7. Conductor material and insulation (Cu/Al, PVC/XLPE).
  8. Future expansion and economy (energy loss cost).

✓ Answer checked by Claude Opus 5.5 (high effort)

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